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  • 1
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    Electrorheological Fluids: Aerospace Applications (1996)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Electrorheological fluids (ERF) are an intriguing class of non-Newtonian industrial fluids. They consist of fine dielectric particles suspended in liquids of low dielectric constants. The objectives of this research were to select a particulate system such that: (1) its density can be varied to match that of the selected liquid, and (2) the dielectric constant of the particles and the liquids should be such that the critical fields needed for asymptotic increase in viscosity are less than or equal to 10 KV/cm. Synthetic Zeolite particles were selected as the solute/suspensions. Octoil oil was selected as the solvent. The results are summarized here.
    Keywords: Nonmetallic Materials
    Type: Proceedings of the 4th Annual Workshop: Advances in Smart Materials for Aerospace Applications; 187-190; NASA-CP-10185
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  • 2
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    Free Volume in Glassy Poly(arylene Ether Ketone)s (1996)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Amorphous polyarylene ether ketones were examined in the glassy state by positron annihilation lifetime spectroscopy ( PALS ) and in the melt by standard rheological techniques. Specimens were well-characterized fractions of two isomeric structures. PALS clearly shows that the polymer with meta linkages in its backbone contains larger voids (greater than 0.25 nm radius). Thus despite their similar bulk densities, the two materials must pack very differently on a local scale. On the other hand, the free volumes inferred from the WLF treatment of melt viscosity data are practically identical in both materials ca. 4% at T(sub g). The comparison between techniques sheds some light on the distribution of free volume.
    Keywords: Nonmetallic Materials
    Type: Investigation of Polymer Liquid Crystals (ISSN 0887-6266); NASA-CR-203249
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  • 3
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    A Method for Characterizing Thermoset Polyimides (1996)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Thermoset polyimides have great potential for successfully meeting tough stress and temperature challenges in the advanced aircraft development program. However, studies of structure-property relationships in these materials have not been very successful so far. Positron lifetime spectroscopy (PLS) has been used to investigate free volumes and associated parameters in a series of variable, segmental molecular weight samples. The free volume correlates well with the molecular weight M(sub c), the cross-link density v, and the coefficient of thermal expansion (CTE) of these materials. Currently, no other techniques are available for direct measurement of these parameters, particularly for polymers in solid phase. Experimental results and their interpretations are presented.
    Keywords: Nonmetallic Materials
    Type: NASA-TM-4707 , L-17515 , NAS 1.15:4707
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  • 4
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    Microstructural Characterization of Thin Polyimide Films by Positron Lifetime Spectroscopy (1996)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Positron lifetimes have been measured in a series of thin aromatic polyimide films. No evidence of positronium formation was observed in any of the films investigated. All test films exhibited only two positron lifetime components, the longer component corresponding to the positrons annihilating at shallow traps. Based on these trapped positron lifetimes, free volume fractions have been calculated for all the films tested. A free volume model has been developed to calculate the dielectric constants of thin polyimide films. The experimental and the calculated values for the dielectric constants of the films tested are in reasonably good agreement. It has been further noted that the presence of bulky CF(sub 3) groups and meta linkages in the polyimide structure results in higher free volume fraction and, consequently, lower dielectric constant values for the films studied.
    Keywords: Nonmetallic Materials
    Type: Polymers for Microelectronics; Apr 05, 1992 - Apr 10, 1992; San Francisco, CA; United States|Investigation of Polymer Liquid Crystals (ISSN 0097-6156); NASA-CR-203249
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  • 5
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    Nuclear Instruments and Methods in Physics Research (1998)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Thermoset and thermoplastic polyimides have complementary physical/mechanical properties. Whereas thermoset polyimides are brittle and generally easier to process, thermoplastic polyimides are tough but harder to process. It is expected that a combination of these two types of polyimides may help produce polymers more suitable for aerospace applications. Semi-Interpenetrating Polymer Networks (S-IPNs) of thermoset LaRC(Trademark)-RP46 and thermoplastic LARC(Trademark)-IA polyimides were prepared in weight percent ratios ranging from 100:0 to 0: 100. Positron lifetime measurements were made in these samples to correlate their free volume features with physical/mechanical properties. As expected, positronium atoms are not formed in these samples. The second life time component has been used to infer the positron trap dimensions. The "free volume" goes through a minimum at about 50:50 ratio, suggesting that S-IPN samples are not merely solid solutions of the two polymers. These data and related structural properties of the S-IPN samples have been discussed in this paper.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-97-208212 , NAS 1.26:208212 , Nuclear Instruments and Methods in Physics Research, Section B: (ISSN 0168-583X); 134; 113-120
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  • 6
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    Morphology of Thermoset Polyimides by Positron Annihilation Spectroscopy (1994)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Thermoset polyimides have great potential for successfully meeting tough stress and temperature challenges in the advanced aircraft development program. However, studies of structure/property relationships in these materials have not been very successful so far. Positron annihilation spectroscopy has been used to investigate free volumes and associated parameters. It has been noted that the free volume correlates well with the molecular weight, cross-link density and thermal coefficient of expansion of these materials. Currently no other techniques are available for direct measurement of these parameters. Experimental results and their interpretations will be discussed.
    Keywords: Nonmetallic Materials
    Type: NASA-TM-111570 , NAS 1.15:111570 , Meeting of the Southeastern Section of the American Physical Society; Nov 10, 1994 - Nov 12, 1994; Newport News, VA; United States
    Format: application/pdf
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